Thermally curable compositions based on condensates of epoxy-functional silanes which contain no urethane groups are known from patent applications EP 1 179 575 A2, WO 00/35599 A, WO 99/52964 A, WO 99/54412 A, DE 197 26 829 A1 or DE 195 40 623 A1. They serve in particular for producing highly scratch-resistant coatings. A key feature is that these known thermally curable compositions must have external catalysts or initiators for the reaction of the epoxide groups added to them (cf., e.g., WO 99/52964 A, page 8, line 29, to page 9, line 20) in order that the compositions cure at a practical rate at comparatively low temperatures of 100 to 160° C.
The use of external catalysts, however, is attended by numerous disadvantages. For instance, it severely curtails the processing time or potlife of the known thermally curable compositions.
Where they are to be used to produce coatings with a thickness of more than 20 μm, as typically employed for the clearcoats of automotive OEM coating systems, they must be modified in order that the resulting coatings do not have stress cracks. This is done, as is known, by incorporating flexibilizing structural elements, which accommodate the stresses, into the three-dimensional, organic-inorganic hybrid networks. In order to bring this about, the known, thermally curable compositions, typically present in aqueous alcoholic media, are admixed with binders which are stable in these media. These binders are preferably in the form of aqueous dispersions. These dispersions, however, frequently exhibit a high level of interaction with the catalysts used, and so they cannot be employed together. The result is the removal of many conceivable possibilities for modifying the known thermally curable compositions in an advantageous way.
German patent application DE 199 10 876 A1 discloses thermally curable compositions based on condensates of silanes of the general formula iXmSiR1-mYn   (i).
In the general formula i the variables X stand for hydrogen, halogen, alkoxy, acyloxy, alkylcarbonyl, alkoxycarbonyl, or —NR′2 (R′=hydrogen and/or alkyl).
The variables R can be identical or different and stand for alkyl, alkenyl, alkynyl, aryl, arylalkyl, alkylaryl, arylalkenyl, alkenylaryl, arylalkynyl or alkynylaryl, it being possible for these radicals to be interrupted by oxygen or sulfur atoms or by the groups —NR′— or —N(H)C(O)O— (urethane) and to carry one or more substituents from the group consisting of halogens and unsubstituted or substituted amino, amide, aldehyde, keto, alkylcarbonyl, carboxyl, mercapto, cyano, hydroxyl, alkoxy, alkoxycarbonyl, sulfonic acid, phosphoric acid, methylacryloyloxy, epoxide and vinyl groups.
The variable Y stands for blocked isocyanate groups.
The indices m and n stand for whole numbers from 1 to 3.
These obligatory silanes can be cocondensed with the optional silanes of the general formula iiXmSiR1-mZn   (ii).
In this general formula ii the index n stands for a whole number from 1 to 4. The variables X and R are as indicated above. The variables Z stand for hydroxyl, amino, NH(CH2)2NH2 or epoxide groups.
There are, accordingly, a massive number of compounds covered by the general formula i which, moreover, can also be combined with a similarly large number of compounds of the general formula ii.
Furthermore, for the known thermally curable compositions it is essential that they are crosslinked primarily by way of the blocked isocyanate groups. For this, however, particularly high temperatures and long cure times must be employed (cf. DE 199 10 876 A1: Example 1, page 3, line 43: 180° C./45 minutes; Example 2, page 4, line 32: 180° C./30 minutes). Conditions of this kind, however, are completely unsuitable for processes in such economically important fields as that of automotive OEM finishing.
Furthermore, although the known thermally curable compositions produce scratch-resistant cured compositions, the chemical stability of the cured compositions leaves much to be desired.
Additionally, the known epoxy-functional silanes and the condensates prepared from them, and thermally curable compositions, frequently contain ether groups, so that as well as an inadequate chemical resistance the cured compositions produced from the known thermally curable compositions also have an inadequate resistance to actinic radiation, especially UV radiation.